This invention relates to compositions of matter and methods of using them to treat scale in various industrial process streams, in particular certain silane based polymers that have been found to be particularly effective in treating aluminosilicate scale in a Bayer process stream.
As described among other places in U.S. Pat. No. 6,814,873, the contents of which are incorporated by reference in their entirety, the Bayer process is used to manufacture alumina from raw Bauxite ore. Because the Bayer process uses caustic solution to extract alumina values from bauxite it is cost prohibitive to perpetually use fresh caustic solution. As a result, a caustic solution known as “liquor” and/or “spent liquor” is recycled back from later stages of the Bayer process to earlier stages and thus forms a fluid circuit. For the purposes of this application, this description defines the term “liquor”. The recycling of liquor within the fluid circuit however has its own complexities.
Raw Bauxite contains silica in various forms and amounts. Some of the silica is unreactive so it does not dissolve and remains as solid sand or mud within the Bayer circuit. Other silica (for example clays and kaolinite) is reactive and dissolves in caustic when added into Bayer process liquors. As spent liquor flows repeatedly through the liquor circuit of the Bayer process, the concentration of silica in the liquor increases eventually to a point where it reacts with aluminum and soda to form insoluble aluminosilicate scale. Aluminosilicate scale comes in at least two forms, sodalite and cancrinite. These and other forms of aluminosilicate scale are commonly referred to, and for purposes of this application define, the terms “desilication product” or “DSP”.
DSP can have a formula of 3(Na2O.Al2O3.2SiO2.0-2H2O).2NaX where X represents OH−, Cl−, CO32−, SO42−. Because DSP has an inverse solubility (precipitation increases at higher temperatures) and can precipitate as fine scales of hard insoluble crystalline solids, its accumulation in Bayer process equipment is problematic. As DSP accumulates in Bayer process pipes, vessels, heat transfer equipment, and other process equipment, it forms flow bottlenecks and obstructions and can adversely affect liquor throughput. In addition because of its thermal conductivity properties, DSP scales on heat exchanger surfaces reduce the heat exchanger efficiency.
These adverse effects require significant downtime of Bayer process equipment, as they must be de-scaled. In addition DSP is difficult to remove and de-scaling requires the use of hazardous concentrated acids such as sulfuric acid.
Typically a “desilication” step in the Bayer process is used to reduce the concentration of silica in solution by precipitation of silica as DSP as a free precipitate rather than as scale. While such desilication reduces the overall silica concentration within the liquor, total elimination of all silica is impractical and changing process conditions within various parts of the circuit (for example within heat exchangers) can lead to changes in the solubility of the DSP and the precipitation of scale.
Some previous attempts at reducing DSP scale in the Bayer process include adding polymer materials comprising three alkyloxy groups bonded to one silicon atom as described in U.S. Pat. No. 6,814,873 B2; U.S. Patent Application Publication Nos. 2004/0162406 A1, 2004/0011744 A1, and 2005/0010008 A2; International Published Application WO 2008/045677 A1; and published article Max HT™ Sodalite Scale Inhibitor: Plant Experience and Impact on the Process, by Donald Spitzer et. al., Pages 57-62, Light Metals 2008 (2008), all of which contents are incorporated by reference in their entirety.
Manufacturing of these trialkoxysilane—grafted polymers however involve unwanted degrees of viscosity and low production throughput due to long reaction times. In addition, synthesis of the polymer described in WO 2008/045677 A1 is difficult as much of it tends to form jelly-like masses, which only dissolve very slowly into a NaOH solution capable of introduction into the Bayer process stream. Other previous attempts to address foulant buildup are described in U.S. Pat. Nos. 5,650,072 and 5,314,626, both of which are incorporated by reference in their entirety.
Thus there is a clear need for and utility in an improved method of preventing or reducing DSP scale formation on Bayer process equipment. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.